The present invention relates to improved silver-glass paste compositions useful for attaching semiconductive elements, e.g. silicon dies, GaAs elements or the like, to appropriate substrates, and to a method for forming electronic components using such pastes.
Prior patents directed to pastes of this general type include U.S. Pat. Nos. 3,497,774; 4,401,767; 4,436,785; 4,459,166; 4,636,254; 4,761,224 and 4,881,974, all hereby incorporated by reference. Typically, these pastes are used for attaching silicon dies to ceramic substrates.
Other improved silver-glass paste compositions are described in U.S. Ser. No. 07/558,327, filed Jul. 27, 1990; Ser. No. 07/248,120, filed Sep. 23, 1988; Ser. No. 07/288,640, filed Dec. 22, 1988 and Ser. No. 07/482,489, filed Feb. 21, 1990, the subject matter of these applications being incorporated herein by reference.
The pastes known in the art typically include the following essential components in the approximate ranges indicated below:
______________________________________ Component % by weight ______________________________________ silver flake 55-80 glass frit 5-25 (e.g. lead borate glass) resin or resin-forming 0.0-2 component (e.g. a methacrylate) organic solvent 5-20 ______________________________________
Other additives, e.g. silver oxide, thixotropic agents, surfactants, or the like also may be included.
In a representative die-attachment process, the paste comprising silver flake, glass frit, resin and solvent is placed in a cavity in a ceramic substrate, the die is placed on the paste and the resulting die/substrate package is fed on a belt through a furnace where the package is heated to remove the organic solvent and sinter the residual silver and glass to bond the die firmly to the substrate. The final bond layer must be completely free of voids. As a consequence, the process usually requires a preliminary drying step in which solvent and other volatiles are evaporated, followed by firing in a furnace to remove residual volatiles and melt the glass.
The preliminary drying step of necessity is quite lengthy, requiring between about 2-10 hours at 60.degree.-80.degree. C., depending on, for size of the die and the surface area of the silver flake particles. Additionally, the ramp rate, i.e. the rate at which the package is fed from the drying step into the furnace, is carefully controlled so as to ensure that organic burnout is at least essentially completed before the sintering of the silver-glass mix takes place. Relatively low temperature ramp rates (e.g. up to 50.degree. C./minute) are very commonly used to ensure solvent removal and optimum results. Belt-type furnaces are normally employed for the firing stage and, depending on the number of temperature zones involved, the dwell time in the furnace can vary from 30-90 minutes or more.
When a large die is bonded, e.g. a die of 400-500 square mils or more, a large amount of bonding agent is needed, and, consequently, a significant amount of solvent and other volatiles must be evaporated. To ensure sufficient evaporation of the volatiles, the drying process may be particularly lengthy and the required ramp rate may be relatively low. The drying time can be decreased by using small silver flakes, which are packed more densely than large flakes and thus permit a faster rate of evaporation. However, when small flakes are used, cracks are likely to form in the bond because sintering tends to proceed too quickly. Thus, effective attachment of a large die to a substrate using conventional methods generally requires the use of large silver particles, and, therefore, a lengthy drying process and use of a relatively low temperature ramp rate.
The silver and glass components in conventional pastes have a tendency to flocculate because of Van der Waals attractive forces, resulting in an increase in free energy of the system when the silver and glass particles are separated from each other. It appears that by using the prior art surfactant additive containing both a lyophobic group which has very little, if any, attraction for the solvent or organic vehicle and a lyophilic group which has a strong attraction for the vehicle, the tendency for flocculation to occur is reduced and paste stability consequently enhanced. Without intending to be limited to any particular theory of operation, it appears that the lyophobic group of the prior art surfactant is adsorbed onto the surface of silver or glass particles to form a steric barrier to the vehicle while the lyophilic portion or "tail" of the surfactant extends into the vehicle or steric layer. Flocculation of particles is inhibited by thickening the steric barrier and physically keeping dispersed particles apart and by reducing the efficiency of interparticle collision. This results in enhanced paste stability with consequent advantages of the possibility of eliminating the drying step, faster heating rates with reduced time to obtain a void-and crack-free bond between silicon die and substrate.